1. Field of the Invention
The present invention relates to inner coatings of metal food containers and packaging. “Inner coating” is understood as meaning a coating which is in direct contact, especially over a prolonged period, with the food or other consumable product present in the coated container.
2. Description of the Prior Art
It is known to use one-component polyurethane stoving lacquers (1-C PUR stoving lacquers), consisting mainly of a blocked polyisocyanate as crosslinking component and a polyol as binder component, as coating compositions for e.g. motor vehicle priming, general industrial lacquering and coil coating. 1-C PUR stoving lacquers are lacquers which cure at elevated temperatures to form a polyurethane network, the hydroxyl groups of a polyol reacting with the blocked NCO groups of a polyisocyanate. On the other hand, the reaction of polyols with NCO groups blocked with CH-acidic esters is a transesterification in which no polyurethane network is formed, so lacquers based on such blocked polyisocyanates are not called 1-C PUR stoving lacquers.
An inherent problem of the use of blocked polyisocyanates for the production of polyurethane coatings is mentioned e.g. in “Polyurethane für Lacke und Beschichtungen”, pages 18 to 35 (M. Bock, Vincentz Verlag, Hannover, (1999): Depending on the type of blocking agent and the stoving conditions, a greater or lesser proportion of the blocking agent remains in free form in the coating. It is also explained that the residual content of blocking agent depends on the particular blocking agent itself and its reactivity.
In the production of coatings of metal packaging, known as can coating, sheets of e.g. tinned iron or aluminium are coated with a stoving lacquer and, after the lacquer has cured, the sheets are processed to the desired commodities. High demands are made on the coating here, especially if it is the inner coating of food containers, for example drinks cans. On the one hand, the lacquers must be flexible enough to withstand deformations during the manufacture of the containers and during transportation of the ready-packed goods without the coating being damaged (e.g. by cracking), and on the other hand the metal substrate must be reliably protected from the effect of corrosive media. In addition, the coating must adhere to the metal substrate, both during deformation and during subsequent sterilization, as is necessary particularly in the case of containers used for packaging foods and drinks. It is undesirable for constituents of the coating to migrate into the packaged goods during manufacture and storage, since this can impair the characteristics of the product.
The state of the art, described in “Internal lacquers of metallic food containers”, K. R. Kumar, Popular Plastics & Packaging, 8/2001, pages 66-70, encompasses the use of coating compositions based on bisphenol A diglycidyl ether (BADGE). It is known that small amounts of this substance, which has a mutagenic action when tested in vitro, can migrate into the packaged foods. It would therefore be advantageous to use BADGE-free coating compositions for the inner coating of cans that are in contact with foods.
The use of polyurethane powder coatings based on polyisocyanates and polyols for the production of inner coatings of packaging containers is described in DE-A 19 545 424. However, it is not known to use polyurethane lacquers based on blocked polyisocyanates for coating the major part of food containers that are in permanent contact with the food. This is attributable to the fact that it is known to those skilled in the art, inter alia from “Polyurethane für Lacke und Beschichtungen”, pages 18 to 35 (M. Bock, Vincentz Verlag, Hannover, 1999), that such coatings have a residual content of free blocking agent. It had to be assumed, therefore, that these chemicals would migrate into the contents and could change or spoil the taste or, in the worst case, damage the consumer's health. The textbook cited above points out that polyisocyanates blocked with diethyl malonate are permitted for the production of crown cork seals. However, although it is known that said compounds do not eliminate the diethyl malonate blocking agent during the crosslinking reaction, a transesterification takes place with the release of ethanol. In view of the relatively small area of a crown cork seal in relation to the overall surface area of the container, and hence the very small amount of ethanol migrating into the food, said ethanol can be ignored, especially in the case of foods and other consumable products that contain alcohol anyway.
Coating systems based on blocked polyisocyanates which eliminate the blocking agent during crosslinking have therefore never been considered for the coating of surfaces that are in direct contact with foods, because the eliminated blocking agents could constitute a potential toxicological risk.
The object of the present invention was to provide coating compositions for coating the inside of preferably metal food packaging that is in permanent contact with foods, which do not exhibit these disadvantages.